Signal transmission system



E. w, KENEFAKE ETAL SIGNAL TRANSMISSION SYSTEM Filed Sept. 17, 1942 2 Sheets-Sheet 1 Inventors: Edwin Kenefake,

' Frank M. Rives, y AAA/A444 Their- Attorney.

Jan. 23, 1945.

E. W, KENEFAKE EI'AL SIGNAL TRANSMI S S ION SYSTEM Filed Sept. 17, 1942 2 Sheets-Sheet 2 TIM IVS/1177' ER RECEIVER In ventors Edwin-W. Kenefa ke,

Fr ian k M. Rives,

Their Attorney.

Patented Jan. 23, 1945 SIGNAL TRANSMISSION SYSTEM Edwin W. Kenefake and Frank M. Rives, Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application September 17, 1942, Serial No. 458,618

3 Claims.

This invention relates, to signal transmission systems, and more particularly to such systems in which signals are transmitted by means of a carrier wave.

In systems arranged to transmit signal modulated carrier waves over a; power transmission line, the carrier wave may be influenced undesirably during its transmission along the power line. Static, and similar electrical disturbances, usually encountered in radio reception, having components of the same frequency as the carrier Wave, are negligibly small on a power transmission line. It has been found, however, that, although the carrier wave is not intermixed in its transmission along the power line with such static and other electrical disturbances, it is attenuated in varying amounts by fluctuating corona losses and the like along the power transmission line. Such variable attenuation changes in large amounts at audible frequency, and introduces an undesirable modulation of the carrier wave.

It has been considered necessary to overcome such undesirable modulation of the carrier wave transmitted along a power transmission line by utilizing a carrier wave transmitter of sufficient power to minimize the effect of this undesirable modulation. This has been only a partial solution, since the modulation produced by such corona losses and the like act to modulate a strong carrier wave nearly as much as a weaker one.

It is an object of our invention to provide a new and improved signal modulated carrier wave transmission system whereby such carrier waves may be transmitted over power transmission lines with minimum interference.

Carrier wave systems are frequently arranged for signal transmission back and forth between two separate stations on the same frequency. Such signal transmission requires that the transmitter at a station be turned off before the receiver is operative to receive a signal from the other station. It is desirable to provide automatic means-t carry out such switching of the transmitters and receivers at the stations, and such automatic switching should be so rapid in response to the presence or absence of signals as to avoid any apparent loss of the beginning of a message. Extremely rap d switching introduces transients in the carrier wave transmission system which are similar to the variable attenuations previously described.

It is a further object of our invention to provide a new and improved signal modulated carrier wave transmission system in which such extremely rapid switching may be used and in which the resulting transients introduced in the carrier wave transmission system produce minimum effeet in the signal channel. 7

It is also' an object of our invention to provide a new and improved signal transmission system in which the signal source is automatically connected to the signal transmission channel in response to the appearance of signals in a minimum time and with minimum loss of signals and minimum effect upon the signals.

The features of our invention which we believe to be novel are set forth with particularity in the appended claims. The invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof may best be understood by reference to the followin description taken in connection with the accompanying drawings in which Figs; la and lb are two parts of a schematic illustration of a signal transmission system embodyin our invention.

In Fig. lb a power transmission line is illustrated at ill as extending between two separate stations, one of which includes a carrier wave transmitter H and a carrier wave receiver 12 connected with a hand set l3, which includes a microphone and a sound reproducer. A similar transmitter and receiver, illustrated in detail, are connected with the power line i U and with a hand set It.

The transmitter thus illustrated in detail comprises a carrier wave generator having an oscillating discharge device 20, and means including a pair of discharge devices 2i and 22 effective to modulate the frequency of oscillations generated by the device 20 in accordance with signals from the hand set it. The carrier wave generated by the device 20 is amplified through a power amplifier device 23 and impressed on the power line it.

The receiver includes electron discharge frequency converter 24, which converts the frequency of received frequency modulated carrier waves to a fixed frequency for amplification in an electron discharge amplifier device 25. The amplified carrier wave from the discharge device 25 is further amplified and its intensity limited to a substantially constant value in another electron discharge amplifier device 26. limited frequency modulated carrier wave from the device 26 is demodulated by a pair of rectifier devices 27 and 28, which reproduce signal voltages from the frequency modulated carrier waves, which signal voltages are amplified through two amplifier discharge devices 29 and 30 and reproduced as sound by the hand set M.

The amplified and Control means is associated with each transmitter and receiver to render the receiver inoperative and start the transmitter whenever signals appearfrom the hand set l4. This control means is also effective to prevent operation of the transmitter and connect the receiver to reproduce signals in the hand set H, in response to the reception of carrier waves from the power line ID. This control means is similar to that described and claimed in the copending application, S. N. 450,499, flied July 11, 1942, by Edwin W. Kenefake for High frequency communication systems. the present control means being arranged and adjusted in a somewhat different and improved manner in accordance with the present invention.

This control means includes a vapor discharge device 40 which is responsive to signals from the hand set to initiate and maintain generation of carrier wave oscillations by the device 20. The device 40 is also effective in conjunction with the electron discharg device 4| and the rectifier device 42 to prevent operation of the carrier wave generator 20 when the frequency converter device 24 and amplifier 25 receive carrier waves from the power line It. Further, the vapor discharge device 40 operating through a relay 43 and an electron discharge device 44 is effective to block operation of the frequency converter device 24 in response to signals from the hand set M. An electron discharge device 45 in the receiver is effective to make the amplifier devices 29 and 30 operative to transmit signals from the rectifler devices 21 and 28 to the hand set M when a carrier wave is received from the power line Ill.

The oscillator including the electron discharge device 20 is a usual type which is made operative when the conductor 50 is connected to ground. The cathode of device 20 is grounded for high frequency current through a suitable condenser 52, and is connected through a resistance 53 to the conductor 50, the resistance 53 being of suitable value to provide a proper operating bias between the control electrode 54 of the device 20 and the cathode 5| when the discharge current of the device 20 flows through the resistance 53. The resistance 53 and condenser 52 are made sufilciently small that, when the conductor 50 is grounded, the cathode 5| rapidly attains a constant positive potential with respect to ground equal to the drop in potential through resistance 53, the time before the cathode reaches this constant potential after the conductor 50 is grounded being in the order of l millisecond or less.

The operating circuit of the oscillator includes a tuned circuit having an inductance 55 and a condenser 56, connected in parallel. The control electrode 54 is coupled to one end of this tuned circuit through a suitable coupling condenser 5'|, and the anode 58 of the device 20 is connected directly to the other end of the tuned circuit. An intermediate tap of the inductance 55 is connected to the positive terminal of a source 59 of operating potential, the negative terminal of which is grounded. By such connection, operating current from the source 59 is supplied through the inductance 55 to the anode 58 -of the device 20 such current flowing through the cathode 5|, and resistance 53 to ground when the device 20 is operating. A suitable grid resistance 60 is connected between the control electrode 54 and the cathode 5| to allow current to flow between the control electrode 54 and cathode 5| during oscillation of the device 20. The screen electrode 5| of the device 20 is connected to an intermediate tap on the source 59, positive in potential with respect to ground, so that connection of the conductor 50 to ground and its disconnection therefrom are effective to make the device 20 and tuned circuit 55, 55 go smoothly into and out of oscillation as the potential of the cathode 5| becomes more or less positive with respect to ground.

Oscillations appearing on the anode 58 of the device 25 are amplified through device 23, whose anode 10 is connected through an inductance H to the positive terminal of source 59. The anode I0 is coupled through a coupling condenser 12 to one terminal of a tuned circuit 13, whose other terminal is grounded. The control electrode 14 of the device 23 is coupled through a serially connected condenser 15 and resistance 16 to the anode 58 of the device 20. The cathode 11 of device 23 is connected to ground through a suitable biasing resistance 18, bypassed by a condenser 19. A suitable grid resistance is connected between the control electrode 14 of device 23 and ground, to provide for the maintenance of suitable bias potential between the control electrode 14 and cathode I1.

Oscillations produced on the anode 58 of the oscillator device 20 are amplified through the device 23, and appear across the tuned circuit 13, of which an intermediate tap is connected through an inductance 9| and a coupling and insulating condenser 92 to the power line It. The inductance 9| is of such value as to resonate with the coupling and insulating condenser 92 at the frequency of oscillations produced by the transmitter.

The frequency of oscillations produced by the oscillator device 20 is varied in accordance with signals from the hand set M by the two discharge devices 2| and 22, so arranged as to simulate reactances. The device 2| is so arranged as to simulate a capacity, and the device 22 is so arranged as to simulate an inductance. Signal potentials from the hand set I4 are applied to the respective control electrodes 93 and 94 of the devices 2| and 22 in pushpull, or balanced, relation, so that an increase of the apparent capacity of device 2| is accompanied with a decrease of the apparent inductance of device 22. The anodes 95 and 96 of the devices 2| and 22 are connected together and to the anode 58 of the oscillator device 29. The cathodes 91 and 98 of the devices 2| and 22 are connected respectively to ground through suitable biasing resistances 99 and I00, each being bypassed with a suitable condenser NH and I02. By such connection, the cathodes 91 and 98 are maintained at the potential of the previously mentioned intermediate tap of inductance 55 of the tuned circuit 55, 56 for currents of carrier frequency. The reactance devices 2| and 22 are therefore effectively in shunt to the major part of the inductance 55, which portion is connected between th anode 58 and the positive terminal of the source 59.

In order to produce a flow of leading reactive current through the discharge device 2|, the control electrode 93 is excited with voltage of carrier frequency from across the condenser 56. To provide this exciting voltage, a phase shifting network including a condenser H0 and a resistance l is connected in shunt to the capacity 56. The condenser I I0 is connected between the control electrode 93 and anode 58, and the resistance ill is connected between the control electrode 93 and, through an Insulating condenser II2, the remaining terminal ,of condenser 56. Consequently, voltage of carrier frequency across the condenser 56 produces a leading current through the condenser III! and resistance III, so that a leading voltage appears across the resistance III, and upon the control electrode 93. With a leading voltage on its control electrode 93, the reactance device 2| draws a leading current through its discharge path, and appears electrically as a capacity connected inshunt to a part of the inductance 55.

In'similar fashion the reactance device 22 is connected'to simulate an inductance. A phase shifting network including a resistance H3 and a condenser I it! is connected in shunt to the condenser 56. The condenser II4 is connected between the control electrode as of reactance device 22 and one terminal-of the condenser 56, and the resistance I I3 is connected between con-- trol electrode 9% and, through a suitable insulatlng condenser 9 I5, to the anode 58.

The arrangement of the reactance devices 2I and 22 is especially desirable, since the currents drawn by these devices are nearly in exact quadrature with the voltage across the tuned circuit 55, 56. Since the voltage appliedbetween the control electrode and cathode in the case of each of the reactance devices 25 and 22 is developed between the above-mentioned intermediate tap of the inductance 55, which is grounded for carrier current, and the intermediate tap of the associated phase shifting network, the voltage developed between these two points, by suitable adjustment of the condenser or resistance in each phase shifting network is effective to make that voltage assume a 90 phase relation with respect to voltage across the tuned circuit 55, 56. It is highly desirable that the reactance devices 2i and 22 draw such purely reactive current since they are thus much more effective to modulate the frequency of the oscillating device 20, especially at low frequencies where carrier current apparatus operates; Atsuch low frequencies it is often difficult to modulate carrier frequency an amount sufliciently large with respect to signal frequencies to obtain satisfactory noise suppression. The arrangement described for the devices 2t and 22 is effective to provide a large shift in the operating frequency of the device 20.

signals from the hand set It are conveyed to the carrier current transmitter and receiver unit along conductors I20, which may be a telephone line if the hand set It is at a long distance from the carrier current apparatus.

The conductors H29 extend from the hand set It to a transformer lZI through which a pair of electron discharge devices I22 and I23 are excited. These discharge devices are arranged in balanced, or push-pull, relation, and their amplified output appears across a pair of serially connected resistances I25 and M in balanced relation. To apply the amplified signal across the resistance I24 to the control electrode 93 ofreactance device 2|, a network is provided including a pair of serially connected resistances I26 and I27, connected between ground and a point between resistance HI and condenser H2. sistances I26 and I2? is coupled through a suit able coupling condenser I28 to a terminal of the resistance E26 on which amplified signal voltage is present. A suitable condenser I29 is connected in shunt to the resistance Hi, and, with the re- A point between the re-,

sistance I26. serves as a filter to prevent the appearance of voltage of carrier wave frequency across the resistance I21. Signal voltage across the resistance I24 is transmitted through condenser I28 to the control electrode 93 since condensers H0, H2 and I29 all have substantially large reactances at the frequencies of the amplified signal voltage.

A similar network is provided to impress amplified signal voltage across the resistance I on the control electrode 94 of the reactance device 22. A pair of serially connected resistances I30 and I3I are connected between ground and a point between the condenser H5 and resistance II3. A point between these resistances I30 and I 3I is coupled through a suitable condenser I32 to a terminal of the resistance I25 on which amplified signal voltage appears. A suitable bypassing condenser I33 is connected in shunt to the resistance I3I, and, with the resistance I30, forms a low pass filter so that voltage of carrier wave frequency does not appear across the resistance HI I. Since the condensers H5, lit and 833 have high reactance at signal frequencies, such frequencies are impressed through resistances I30 and H3 on the control electrode 94 in opposite phase to signal voltages onthe control electrode 93.

It is highly desirable that means be provided to modulate the frequency of the generated carrier wave by an amount which is about equal to the highest signal frequency to provide substantial suppression of various types of undesired voltage which are introduced into the signal generated in the transmitter and transmitter to the receiver. It has been recognized that frequency modulation is highly desirable for the radio transmission of messages, but it has not been recognized that signal transmission on a power 40 line by carrier current is susceptible of great improvement by the use of frequency modulation, and especially by the use of frequency modulation where the frequency shift is not much, if any, greater than the highest signal frequency. A

4:, large :part of the interfering. or noise, voltage in a radio receiver, especially when the receiver is' tuned to a weak signal, is due to energy whose periodicity lie within the band of frequencies to which thereceiver is tuned, Noise voltage present on a power line presents no great difficulty under normal conditions because carrier energy levels on power lines are usually high enough that noise voltage present causes little interference. However, the carrier current is subject to certain influences, such as corona, which change it intensity in undesirable fashion. Furthermore, there are cases where the attenuation of a line is high and the received signal is low so that noise voltages may cause considerable interference. We have also determined that the transmission of signals by the frequency modulation of such carrier currents over power lines is effective to reduce the interference in such cases. It is desirable, in order that these influences be reduced to a minimum, that modulation of the frequency of the carrier current transmitted over the power line be produced in which all) the shift of frequency is about the same as, or.

rangement is especially effective in modulating the frequency of the carrier wave generated by the oscillator device 20 over a sufficient range of frequencies at the low frequencies used over power lines.

It is highly desirable for another reason that,

ceiver started in response to carrier current on.

the power line I0. In order that these control circuits shall operate at extremely high speed, so that conversation between two stations along the power line I may most nearly approximate normal telephone conversation, the effect of the large surges introduced in the transmitter and receiver by such rapid switching action must be minimized. The use of frequency modulation of the carrier; wave for the transmission of such signals is highly desirable in order that these switching surges may be effectively eliminated.

In order to receive sig als at relatively low signal-to-noise ratio, it is of great importance that the frequency modulation of the carrier wave shall not greatly exceed the highest frequency component of the audio signal. It is especially desirable to achieve, not the maximum amount as the input to the device 26 from the tuned circuit I is above a certain small minimum value.

The output of the amplitude limiter device 26, being of substantially constant amplitude, is demodulated in a suitable frequency discriminator circuit, such as that described and illustrated in Patent 2,121,103, Seeley, issued June 21,.1938. Detected signals, which correspond to the frequency modulation of the carrier current from the amplitude limiter device 26, appear across the serially connected resistances I and I6I of the discriminator circuit. These resistances are respectively connected in shunt to condensers I52 and I53 whose reactances are low at the frequency of the frequency modulated carrier current. It should be particularly noted that the resistances I50 and I5I are nowhere connected to ground, as in the above-mentioned Seeley patent, but that they are arranged so that signal voltage appears thereacross in balanced, or pushpull, relation with respect to ground.

The circuit arrangement by which the signal voltage across the resistances I50 and I5I is made to appear in balanced, or push-pull, relation, includes apair of serially connected resistances I54 and I connected in shunt to the resistances I50 and I5I, and having a point between such resistances I54 and I55 connected to ground. The point between the resistances I50 and I5I is bypassed to ground for carrier frequency current through a bypassing condenser I56. By such a circuit arrangement the detected signal voltages of noise suppression, but a readily distinguished signal even though undesirably influenced in its transmission along the power line.

It is preferred that frequency modulation of the carrier wave shall not be less than the highest frequency component of the signal to be transmitted, since a smaller amount of frequency modulation actually reduces the highest frequency component of the signal which can be transmitted. It is at present believed, furthermore, that the maximum absolute carrier Wave intensity at the receiver is achieved when the amount of frequency modulation is about the same as, or not much greater than, the highest frequency component of the signal to be transmitted.

The receiver associated with the transmitter including the oscillating device 20, as explained previously, includes a frequency converter device 24, an intermediate frequency amplifier 25, a limiting amplifier 26, a frequency discriminator circuit including diode detectors 21 and 28, and an audio amplifier including a pair of discharge devices 29 and 30. The frequency converter device 24 and its associated circuits, as well as the amplifier device 25 and its associated circuit, are similar to analogous apparatus in the receiver described in the above-mentioned copending'application of Edwin W. Kenefake, Serial No. 450,449, filed July 11, 1942. The output tuned circuit I40 of the amplifier device 25 is coupled to the input tuned circuit I4I of the amplitude limiter device 26, in usual fashion. The control electrode I42 of the device 26 is connected to one terminal of the tuned input circuit I4I,-the other terminal of the input circuit I4I being connected to ground through a suitable grid resistance I43, shunted by a condenser I44. The condenser I44 has relatively small reactance at the frequency of waves for which the circuits I40 and M! are tuned, and the resistance I43 is of sufficiently large value that the output of the amplitude li-miter device 26 is substantially constant so long across the resistances I50 and I5I exist across the resistances I54 and I55 in balanced relation.

The power amplifiers 29 and 30 have their cathodes connected together, and through a resistance I60 to a suitable tap on a source IBI of potential, the tap being at a positive potential with respect to ground. The source I6I is provided to supply operating current to the receiver including the frequency converter device 24, the amplifier device 25, the limiter device 26, and the power amplifiers 29 and 30 and control device 45. The anodes I62 and I63 of devices 29 and 30 are connected to opposite terminals of the primary of the transformer I64, the center tap of the primary of transformer I64 being connected to the positive terminal of the source I6I, whereby discharge current is supplied to amplifiers 29 and 30. The secondary of transformer I64 is connected across the conductors I20 so as to supply amplified signals to the hand set I4 from the receiver.

Detected signals across the resistances I54 and I55 are supplied across two serially connected voltage dividing resistances I65 and I66 through coupling condensers I61 and I68. The control electrodes I69 and I10 of the devices 29 and 30 are connected to respective adjustable taps on the voltage dividing resistances I65 and I66. A point between the voltage dividing resistances I65 and I66 is connected through a resistance IlI, con nected in shunt to a condenser I12, to the same intermediate tap of the source I6I to which the cathodes of the devices 29 and 30 are connected through the resistance I60. When current is not flowing through the resistance III, the discharge current of the amplifiers 29 and 30 flowing through the resistance I60 provides a suitable operating bias potential for the control electrodes I69 and I10 of the amplifiers 29 and 30, so that the balanced signal potentials across the re-' sistances I54 and I55 are amplified through the amplifiers 29 and 30 and supplied through transformer I64 to hand set I4.

The control device 45 is so arranged as to produce a flow of current through resistance III whenever there is no carrier wave in the tuned circuit MI, or whenever the intensity of the car rier wave in that tuned circuit is'below a desired minimum intensity. To this end the anode I80 of the control device 45 is connected to a point between the resistances I65, I86 and. Ill, and the cathode I8I is grounded. The control electrode I32 is connected through a resistance I83 to a point between tuned circuit I 4I and resistance I43, and this control electrode I82 is also connected to ground through a condenser I84.

In operation, this control device 45 acts to cut oil the discharge current in the amplifiers 29 and 30 at all times except when a satisfactory detected signal exists across the resistances I54 and I55. Unless a carrier wave of sufficient intensity exists in the tuned circuit Hi, the limited-device 26 does not operate. That is, its control electrode I42 is not driven to sufficiently positive potentials to cause self-biasing action for the device across the resistance I43 shunted by the condenser I94.

When carrier waves of sufiicient intensity exist in the tuned circuit MI, such self-biasing action takes place, by rectification of the carrier wave on the control electrode M2 of the device 25, with corresponding production of a negative bias potential across the resistance M5. The negative bias potential across the resistance I 43, developed when a carrier wave of suflicient intensity exists in the tuned circuit l ll, is sufiicient, when. applied to the control electrode I82 of the control device 45, to reduce the discharge current in that device to zero. As explained above, when the device 95 is not transmitting any discharge current, so that no current is flowing through the resistance ill, the power amplifiers 29 and 30 are operative. When there is no carrier wave in the tuned circuit Ml, or a carrier wave of less than a certain intensity, little or no negative bias voltage appears across the resistance I43, and the control device i5 passes a substantial discharge current, which flows through the resistance Ill, and places a, large negative bias potential on the control electrodes I69 and H0 with respect to the cathodes of the power amplifiers 29 and 30, so as to cut oil the flow of discharge current in those amplifiers.

The control arrangement shown in Fig. 1a is identical in structure, although somewhat different in adjustment, with the control apparatus shown in Fig. 1a of the above-mentioned copending application, S. N. 450,449 of Edwin W. Kenefake, filed July 11, 1942. Briefly, when sound impinges on the hand set I4, such sound signals are transmitted through the conductors I20 and through conductors I90 in Fig. 1a togthe transformer I9I. These sound signals from transformer I9I are amplified through discharge device I92 and impressed on the control electrode I93 of the vapor discharge device 40. The sound signals appearing on the control electrode I93 are suflicient to overcome a normally maintained negative bias potential on that control electrode and cause the vapor discharge device to become conductive. Since the anode'I94 of the vapor discharge device 40 is connected through the movable switch member I95 of relay 43 to conductor 50, which conductor in the absence of signals from the hand set I4 is maintained at a high positive potential by connection to the operating coil I95 of relay 43 and resistance I9? to the positive terminal of the source I98 of po- III through the vapor discharge device 40, and the relay 43 is simultaneously energized. Connection of the conductor 50 to ground, as explained previously, is effective to start operation of the trans-. mitter within a very short time interval determined by the size of resistance 53 and condenser 52, and since sound signals are impressed on transformer I2I from hand set I4, the carrier wave generated by the oscillation device 20 is modulated in frequency in accordance with such sound signals and transmitted over the power line I0 to distant stations. I

Energization of the relay 43 subsequent] causes its operation, and causes the direct grounding of conductor 50 through the movable switch contact I95, the vapor discharge device 40' thereafter having only the duty of maintaining relay 43 energized as long as signals appear from the hand set I4. Closure of the movable switch contact I98 of the relay 43 is effective to reduce the normally maintained negative bias potential on the control electrode I93 of vapor discharge device 40, whereby it is made easier for signals from the hand set I4 to maintain the'device 40 conductive. Closure of the movable switch contact I99 of the relay 43 is effective to close a circuit for the relay which prevents its opening after deenergization for a predetermined time adjustable by adjustment of a variable resistance 200, all as explained in the above described application. Furthermore, closure of the movable switch contact.20l of relay 53 is effective to apply alternating potential from a suitable source 202 through the anode I94 of the vapor discharge device 40, whereby the anode potential of the device 40 is periodically reduced to a low value such that it can become nonconductive when signals from the hand set I4 cease.

When the conductor 50 is grounded through the vapor discharge device 40 or directly, through the movable switch contacts I95 of the relay 43, the double triode discharge device 44 is efiective, as explained in the above described application, to produce a negative potential across the resistance 203, with which a condenser 204 is connected in shunt. Also, as explained in that above mentioned application, the oscillator and first mixer control electrode 205 of the device 24 are connected through a resistance 206 to the negative potential terminal of the resistance 203, the other terminal of resistance 203 being grounded.

In operation, the frequency converter device 24 is blocked by the negative potential applied from the resistance 203 to its control electrode 205 when signals appear from the hand set I4 in the shortest possible time. That is, the operation of the amplifier I92, the vapor discharge device 40, and the discharge device 44 in producing a negative bias potential across the resistance 203, and the application of that negative bias potential to the control electrode 205, is caused to take place in a relatively short time. For this purpose, the discharge device 44 should have low internal resistance when it is conductive, and the negative potential source from which the potential across resistance 203 is derived through the discharge path of the device 44 must have a low resistance.

After the receiver is blocked by the negative potential applied to the control electrode 205 in response to the appearance of signals in the hand set I4, the transmitter is turned on. The com denser 52 and resistance 53 are so proportioned tential, the conductor 50 is effectively grounded potential.

as to cause the desirable amount of'time delay in turning on the transmitter. It is desirable that blocking of the receiver takes place as quickly as possible so that the time of turning on the transmitter may be minimized, resulting in minimum loss of the initial one or two cycles of the signals from the hand set I4. Such rapid blocking of the receiver is not effective to produce a surge which is amplified through the power amplifiers 29 and 30, because the discriminator circuit including the diodes 21 and 28 is balanced and cooperates with the limiter device 26, so that the receiver does not reproduce such a surge, even if the amplifier devices 29 and 30 were made operative by action of the control device 45. Such fast starting of the transmitter does produce a surge in the carrier wave transmitted by it, but the receiver is connected to the power line I which receives this carrier wave and the surge produced in it by rapid starting of the transmitter are arranged as the receiver shown-in detail in Fig. 1b, and include limiter devices and balanced frequency discriminators,

and are therefore not effective to reproduce a signal from such surges. These receivers are,- however, effective to reproduce frequency modulation of the carrier wave, which is initiated immediately when the carrier wave is transmitted, since signals which start the transmitter. from the hand set I4 immediately begin modulation of the frequency of the carrier wave.

When signals from the hand set I4 no longer appear, vapor discharge device 40 becomes nonconductive, and after a considerable time greater than that which usually elapses between parts of a single message, the relay 43 opens. The time delay after deenergization of the relay 43 before it opens may be of the order of 150 milliseconds. After the relay 43 opens, the carrier wave is no longer generated by the transmitter, the oscillator device20 being deenergized, and that section of the device 44 which maintains the negative bias potential across resistance 203 also becomes non-conductive. Thereafter condenser 204 discharges slowly through the resistance 203, and eventually returns the control electrode 205 of the frequency converter device 24 to ground The discharge time of the condenser 204 through resistance 203 may actually be fairly rapid, it being necessary only to assure that the frequency converter device 24 remain blocked until the oscillator device 20 has stopped oscillating. The discharge period for condenser 204 may be in the order of a few milliseconds.

When signals are not appearing from the hand set I4, so that the transmitter is turned off and the frequency converter device 24 operative, signals may appear from the power line I0 and be received by the receiver so as to block the transmiter and reproduce such signals in the hand set I4. When such signals are received and appear in the tuned circuit I4I, they are impressed through a couplingcondenser 2 I 0' upon the rectifier device 42, wherein they are detected to produce a continuous potential across the resistance 2 shunted by the condenser 2I2. carrier'current is received by the receiver, the continuous'potential remains across the resist-- ance 2H and is amplified through the double triode discharge device 4| to impress a negative bias potential on the second control electro'de'2l3 of the vapor discharge device 40. All this action is similar to that described in the above mentioned application, S. N. 450,499, and is made to take place in the minimum possible time. As explained previously, when such signals appear in the tuned circuit I4I, the control device 45 operates to make the power amplifiers 29 and 30 reproduce detected signals across the resistances I54 and I55 in the hand set I4.

The time after the appearance of a negative bias potential across the resistance I43 until the control device 45 becomes non-conductive is very short, being determined by the condenser I84 and resistance I83. It is necessary thereafter for the condenser I12 to discharge through the resistance Ill, and only after such discharge are the amplifier devices 29 and 30 made operative. The time for such discharge is suilicient to insure that the vapor discharge device is blocked, to prevent operation of the transmitter, before the amplifier devices 29 and 30 are made operative. The time after signals from the power line I0 are received by the receiver until the control device operates to make the power amplifiers 29 and 30 operative should be a minimum. The arrangement of the amplifiers 29 and 30 in a balanced circuit insures that the action of the control device, 45 in making themoperative or in- So long as operative has no effect of itself on the hand set I4. Such high speed operation is possible be cause there is no necessity for maintaining the amplifier devices 29 and 30 non-conductive until surges received from the power line It die out, since such surges are not detected by the receiver because of the use of frequency modulation with the limiter device ,26 and the frequency discriminator circuit including the diodes 21 and 28. Since such frequency modulation arrangements are utilized, it is possible to make the receiver operative long before received surges die out, and the use of the balanced power amplifier stage makes it possible to put the receiver into operation at such high speed, because that balanced stage does not reproduce switching surges caused in the receiver for reproduction by the hand set I4.

After signals from the power line Ill cease, the bias potential across resistance I43 disappears, and the control device 45 becomes conductive immediately to block operation of the amplifier devices 29 and 30. Immediately thereafter, in a time determined by the time constants of the resistance 2H and condenser 2I2, the negative bias potential on the control electrode 2I3 of vapor discharge device 40 is allowed to disappear leaving the transmitter in condition such that it may be started if signals appear from the hand set I4.

It is only by the 'integrated use of such an ultra high speed control system together with frequency modulation circuits which effectively do not respond to switching surges that our carrier wave transmitter and receiver are able to operate so rapidly and with so little loss of the beginning of a signal that conversations may be carried on between stations including such apparatus with as much facility as over a normal telephone line. As explained previously, the transmission of the signals by modulation of the frequency of the carrier waves is of great additional advantage in reducing the peculiar type of interference with the transmission of a carrier wave experienced along a high voltage power line. By the use of this frequency modulation in which the carrier 'wave frequency is shifted over a band of frequencies whose width is about the same a the highest signal frequency, a very great reducti0n in the carrier wave power necessary to produce an intelligible signal at the receiver is achieved.

of the signal, less than 1 as much power is necessary to transmit a signal than if the signal were transmitted by amplitude modulation of the carrier wave. This reduction in power may be made even more favorable if pre-emphasis is utilized. That is, if higher signal frequencies are made to shift the frequency of the carrier wave a greater amount than lower signal frequencies, it has been found that about fifty times as much power is required to transmit a signal of the same intelligibility by amplitude modulation of the carrier.

While we have shown and described a particular embodiment of our invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from our invention in its broader aspects, and we therefore aim in the appended claims to cover all such changes and modification as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

i. In a system for transmitting signals over an electric line subject to disturbing influences of the nature of that due to corona, the combination of a carrier wave source, a signal, source, means for modulating the frequency of a carrier wave from saidcarrier wave source in accordance with the intensity of signal from said signal source and for. impressing such frequency modulated carrier waves on said line, and receiving means for receiving said frequency modulated carrier wave from said line and reproducing said signal from the modulations of frequency of said carrier wave, said carrier wave being subject to undesired signal frequency modulation of intensity of the nature of that due to corona during its passage along said line, said receiving means including means for preventing the reproduction along with said signals of such modulations of intensity of said carrier wave.

2. In combination, an electric line subject to disturbing influences of the nature of that due to corona, a carrier wave source, a signal source, means for modulating the frequency of a carrier wave from said carrier wave source in accordance with the intensity of signals from said signal source and for transmitting said modulated carrier wave over said line, means for receiving said modulated carrier wave from said line and for reproducing signals in accordance with the frequency modulation'of said carrier wave, said carrier wave being subject to undesired signal frequency amplitude modulation in its passage along said line of the nature of that due to corona, said receiving means including amplitude limiting means for said carrier wave and a balanced frequency discriminator for detecting frequency modulation of said carrier wave and the modulation of the frequency of said carrier wave being of the same order as the highest frequency component of said signal whereby minimum carrier wave power is required to transmit said signal with satisfactory intelligibility from said signal source to said receiving means.

3. In a carrier current system for an electric line having the characteristic of introducing noise by causing signal frequency amplitude modulation of the carrier of the nature of that due to corona, means for impressing on the line a carrier modulated in frequency by a signal, and means for receiving said carrier from said line and for reproducing said signal from the modulations in frequency of the carrier, said receiving means including means for rejecting amplitude modulation of the carrier. I

EDWIN W. KENEFAKE. FRANK M. RIVES. 

